iMac 21.5 2012 SSD Question

I am just curious if anyone has put an SSD in a 21.5" 2012 iMac. I've found conflicting reports on the internet as to whether or not it's even possible. I don't have a Fusion Drive in mine (just the 1 TB spinning drive) and I'm beginning to regret that decision a bit. I know some people have suggested putting the OS on an external SSD. Is it possible to open up the 21.5" and place one internally (or pay someone who knows what they're doing to do it)? Has anyone tried the external option?